Fluid flow measuring devices and methods are known for use with fluid flow systems to measure pressure of a fluid flow, which may be converted into a fluid flow rate. A pitot tubes, for example, is a device commonly used to measure pressure of a fluid flowing through a fluid line. The pitot tube may be positioned to measure fluid pressure at a specific point within the fluid line, and typically includes an opening that may be placed in the stream of the fluid flow. To obtain an accurate reading, the pitot tube must be positioned so that the opening is located in the exact center of the stream and oriented parallel to the fluid flow path. A gauge may be operably coupled to the pitot tube to provide a pressure reading.
In a pitot tube, static pressure, which is atmospheric pressure in an open system, is compared to total pressure. Using Bernoulli's Equation, the pressure differential is then translated to a flow velocity. A gauge device may be graduated such that the readout is directly displayed as a fluid velocity.
Pitot tubes are commonly used where there is a desire to determine the flow rates of fluids for equipment testing. Fire pumps, in addition to other pumping equipment, often must be tested to ensure the equipment meets certain performance specifications, and therefore it is important to provide accurate devices and methods for determining fluid flow rates.
Many fire hydrant flow tests are conducted by taking a pitot reading directly from the nozzle on the fire hydrant. Due to inexact orifice diameter, excessive turbulence (which may cause fluctuations in the observed pressure level of +/−10 psi), and incorrect pitot tube positioning, these options give the least dependable readings.
Some devices integrate the pitot tube into the fluid line. A general problem with pitot tubes is that they are difficult to position to obtain accurate pressure readings. As noted above, the pitot tube must be positioned in the center of fluid flow and oriented parallel to the flow. As the pitot tube placement deviates from these requirements, the less accurate the pressure readings will be. Additionally, the pitot tube structure extends partially into the fluid line, and therefore is susceptible to damage by solids entrained in the fluid.
Conventional pitot tubes and other flow testing apparatus are also susceptible to inaccuracies due to turbulent fluid flow. Turbulence may be generated by valves, elbows, or other components that disrupt the flow of fluid through the pipe. Consequently, conventional devices typically require minimum lengths of straight pipe upstream and downstream of the flow testing apparatus to reduce turbulence and therefore improve accuracy of the data obtained by the flow testing apparatus.
More recently, the present applicant developed a pitotless nozzle disclosed in U.S. Pat. No. 6,874,375 to Grenning, which issued on Apr. 5, 2005. The pitotless nozzle includes a constant pressure nozzle that promotes laminar flow through the nozzle. Laminar flow has a relatively constant pressure profile, and therefore an accurate pressure reading may be obtained from a periphery of the flow, without requiring components that obstruct the fluid flow. While the pitotless nozzle of the '375 patent represented an improvement over prior devices, the range of flow-rates that it could measure was limited and improved accuracy of pressure measurements used to calculate flow-rates was still desired.